In the electrochemical production of propylene oxide, propylene is converted to propylene halohydrin by reaction with halogen generated in situ by the anodic oxidation of the halide salt of an alkali metal in aqueous solution. The propylene halohydrin is converted to propylene oxide by reaction with the hydroxyl group at the cathode from which hydrogen is liberated. The general scheme of reaction when sodium bromide is used as the electrolyte is: ##EQU1##
In principle, only water, propylene and electrical energy are consumed in the formation of propylene oxide and hydrogen. The halide electrolyte, sodium bromide, is continuously oxidized and regenerated within the cell for further use, although losses of bromine may be caused by the formation of hypobromite and bromine gas.
The advantages of this electrochemical route, which obviates the production of waste calcium chloride encountered in the conventional chemical process, have long been recognized but attempts to implement it have not proved to be very effective.
French Patent Specification No. 1,375,973 and W. German Auslegeschrift No. 1,258,856 proposed the use of diaphragm cells in which propylene halohydrin is generated at a porous anode and passes through the diaphragm into an alkaline catholyte in a porous cathode where it is saponified to propylene oxide. However, these cells are complex and the efficiency low.
U.S. Pat. No. 3,394,059 proposed carrying out the halohydrin process in a non-divided cell, preferably a flowing mercury cathode cell, in which propylene was simply bubbled into the electrolyte. Again, the performance was poor and F. Beck (IUPAC XXIVth International Congress, Hamburg, 1973, Vol. 5, "Applied Electrochemistry", pages 111-136) has claimed an improved performance using a capillary gap cell. In this, propylene dispersed in a dilute NaBr electrolyte is supplied through a central hole in a pile of electrode discs and flows radially outwards between the discs. Th gap between the electrode discs was made small (0.2 to 0.5 mm) to enable low bromide concentrations to be handled with low ohmic losses. A current efficiency of 70% or just above and an energy consumption of 0.23-0.30 kwh/gmol propylene oxide are reported for a small capillary gap cell, but scaling up this cell for industrial production would involve difficulties.
Fleischmann et al. (Symposium on Electrochemical Engineering I, Newcastle 1971, Editor J. D. Thornton) have studied the synthesis of propylene oxide using a bipolar packed bed cell. The cell consisted of a packed bed made up of a mixture of conducting and non-conducting particles. The conducting particles become biplar by using dilute electrolyte in the cell and applying sufficient voltage gradient between the contact electrodes so as to overcome the resistance drop in the electrolyte. Using glass coated with graphite as the conducting particles and glass beads as the non-conducting particles, all particles having a diameter of about 0.05 cm, the energy consumption of such a cell was found to be high, in the range of 2.5-3 kwh/gmol propylene oxide.
A bipolar rod flow cell was used by King et al. (Trans. Inst. Chem. Eng., 53, 1975) for the production of propylene oxide. The cell consisted of vertical rows of electrically-conductive rods, separated from one another by a small gap. The electrolyte was fed to the top rods, flowed downwards over the vertical rows and was collected from the bottom rods for recirculation. The gaseous reactant, propylene, was passed up the space between the vertical rows, in continuous contact with the electrolyte film. The current efficiency of this cell was of the order of 70% and the energy consumption is estimated in the range 0.35-0.4 kwh/gmol propylene oxide.
R. E. W. Jansson et al. have developed a bipolar electrochemical pump cell for which an energy yield below 0.2 kwh/gmol of propylene oxide is claimed (Journal of App. Electrochemistry, 7, (1977), 437-443) for trial experiments on a laboratory scale using a cathode rotating at 3000 rpm with an electrode gap of 0.25 mm. However, the structure is not easily scaled-up for industrial production.
Various other cell structures designed to provide a gas supply to the electrolyte are also known. For example, in the electrowinning of metals such as copper, it is well known to supply a gas through bubble tubes situated below the electrodes in order to agitate the electrolyte (e.g. see U.S. Pat. No. 3,875,041 and the earlier patents referred to therein). Another suggestion made in U.S. Pat. No. 3,259,049 was to provide electrolyte agitation in an electroplating tank using a hollow, flat manifold which is placed in the electrolyte, under the electrodes, and has a perforated upper surface for bubbling gas up into the electrolyte, the gas being supplied to the manifold via a gas flow tube. In contrast to the fixed bubbletube arrangements, the entire manifold structure was made removable to facilitate periodic cleaning to remove fragments which may block the perforations in the manifold.